>>Yup. And there are two doors available. ONE choice between TWO doors (options). The fact that there were three doors at the start simply cannot influence ONE choice between TWO options in the second round.

When you first pick a door, you have a 1/3 chance of picking the car.
Yes?

If I now open a goaty door, the probability is still 1/3 I have a car.
Yes?

So if I have two options, and the option I've got is 1/3, the other must be 2/3.
Yes?